The present invention relates generally to packet data communication networks and, more particularly, to a method and apparatus for compensation for antenna diagram optimization when a transmitted message contains information for more than one spatially separated mobile.
The growth of commercial communication networks and, in particular, the explosive growth of cellular radiotelephone networks, have compelled network designers to search for ways to increase network capacity without reducing communication quality beyond consumer tolerance thresholds. At the same time usage of mobile communication equipment for transmission of data rather than speech has become increasingly popular by consumers. The possibility to send and receive electronic mail and to use a web browser to obtain world-wide-web access is frequently discussed as services that will be more and more used in wireless communication networks. As a response to this, communication network designers search for ways to efficiently transfer data information to and from mobile users.
There are fundamental differences between requirements for data communication and speech communication. For example, data communication can have a number of different service classes with different requirements on delay and error while speech has a constant high demand on delay and a moderate demand on error. The use of packet data protocols, which are more suitable for transmission of data than circuit switched protocols, have begun to find its way into cellular communication networks. Packet service integration in both GSM cellular networks as well as TDMA (IS-136) cellular networks is presently being standardized.
Today, GSM networks provide a circuit switched data service, which can be used to interconnect with external data networks. The circuit switched data service is used for both circuit switched as well as packet switched data communication. To make packet switched data communication more efficient, a new packet switched data service called GPRS (General Packet Radio Services) and an extension known as EGPRS have been introduced as a part of GSM. EGPRS/GPRS allows for packet switched communication, e.g., IP or virtual circuit switched communication. EGPRS/GPRS supports both connectionless protocols (e.g., IP) as well as a connection-oriented protocol (X.25). One of the advantages with a packet switched data communication protocol is that a single transmission resource can be shared between a number of users. Thus, in the case of, e.g., a GSM cellular network, a timeslot on a radio frequency carrier can be utilized by several mobile users for reception and transmission of data. The shared transmission resource is managed by the network side of the cellular network both for downlink and uplink transmissions.
In EGPRS/GPRS networks, to share transmission resources between a number of users, the network uses temporary flow identities (TFI) and uplink state flags (USF). When starting a transmission a mobile is assigned one or more timeslots in the uplink and/or downlink. In the assignment of timeslots the mobile is assigned a TFI and/or a USF. The TFI is attached to the data blocks transmitted in the downlink to indicate the destination of the particular data blocks. Accordingly, all mobiles listen to the assigned timeslots in the downlink and try to decode all blocks transmitted on the downlink. After decoding the received blocks a mobile will check the TFI for the particular block to determine if the mobile is the destination of the particular block.
FIGS. 1A and 1B illustrate the scheduling of the uplink among a plurality of mobiles in a EGPRS/GPRS network. Assume that mobiles A and B are assigned to shared packet data channels (PDCHs) TN1 through TN3. As discussed above, each mobile will listen to the assigned timeslots to try to decode all blocks transmitted on the downlink. While attempting to decode the blocks transmitted on the downlink, the mobile will also look for an indication of whether it is allowed to transmit on the uplink. In EGPRS/GPRS the USF provides this indication. The USF indicates to a mobile that it is allowed to transmit an uplink block which corresponds to the downlink block that contains the USF. Referring again to FIG. 1A, mobile A will detect that its USF is included in the downlink radio block containing data for mobile B. Accordingly, as illustrated in FIG. 1B, mobile A transmits in the next uplink block. It will be recognized that in EGPRS/GPRS there are two modes for dynamic allocation of timeslots, granularity 1 and granularity 4. In granularity 1, a USF indicates the allocation of one uplink block (4 bursts) as illustrated in FIG. 1. With granularity 4, a USF indicates the allocation of four consecutive uplink blocks (16 bursts). Thus with granularity 4 the mobile only has to check for the USF in one quarter of the downlink blocks.
One method for reducing downlink interference is to use an adaptive antenna system. In general, an adaptive antenna system is able to adapt to its characteristics to changes in the network. One of the more important features of adaptive antenna systems is that the base station is able to detect the direction to all mobiles in the cell. The base station then adapts its radiating pattern for each mobile in order to optimize the transmission. In a switched beam system, the base station transmits information which is intended solely for the specific mobile using a narrow antenna beam which is directed to the mobile. Using a narrow antenna beam minimizes interference by not radiating a large amount of energy through an entire cell when a mobile is known to be located in a particular portion of the cell. This minimization of interference is known as antenna diagram optimization.
FIG. 2 illustrates an antenna diagram of the beams and the sector antenna of an exemplary adaptive antenna system. The antenna illustrated in FIG. 2 has eight fixed beams which cover smaller portions of a sector of a cell. Further, the antenna includes a sector antenna which covers the whole sector and envelopes the areas covered by the eight fixed beams. Accordingly, if information is to be transmitted to a single mobile, the mobile network selects the beam which covers the particular portion of the sector that the mobile is located in. If, however, the same information needs to be transmitted to all of the mobiles in a sector, e.g., control channel information such as a broadcast control channel (BCCH), then the sector antenna is employed. Using the individual beams to transmit to mobiles reduces the interference caused throughout the cellular network and in turn allows tighter frequency reuse and/or provides higher network capacity.
In addition to the above described advantages of adaptive antenna systems, employing adaptive antenna systems in a EGPRS/GPRS network will produce a significant throughput increase. This increase in throughput is due to the link quality control (LQC) features of EGPRS/GPRS. In EGPRS/GPRS a number of different coding schemes and two modulations are used to ensure a maximum throughput at a specific radio link quality, which may be measured in terms of a carrier-to-interference ratio (C/I). In GPRS a specific coding scheme is called CS and there exists CS1-CS4. In EGPRS a specific combination of modulation and coding is called MCS and there exists MCS1-MCS9. To implement LQC in a EGPRS/GPRS network, a mobile reports the mean and variance of the block error rate (BER) of the downlink transmission to the radio network. The radio network uses these reports to select a CS (in a GPRS network) and an MCS (in an EGRPS network) which has the best balance of radio channel throughput and a low BER.
FIG. 3 illustrates the carrier distribution function for throughput gain for a network with a ⅓ reuse pattern, i.e., a resuse pattern where every third cell uses the same frequencies. The curve with the circles illustrates the throughput distribution for the system deployed with sector antenna and the curve with the plus signs illustrates the throughput distribution of the system deployed with adaptive antenna. As illustrated in FIG. 3, the throughput gain for a network employing adaptive antennas is between 10 kbps and 20 kbps for every timeslot for every level on the carrier distribution function.
FIG. 4 illustrates two mobiles in a EGPRS/GPRS networks which uses adaptive antennas. As illustrated in FIG. 4, mobile A is located in beam 410 and mobile B is located in beam 420. Assume now that mobile B is to receive downlink data while mobile A is to receive a USF. As described above, in EGPRS/GPRS the USF is included in a data block transmitted in a downlink timeslot. However, since the data block contains both the USF for mobile A and the data for mobile B and since mobile A and mobile B are located in different antenna beams, mobile A will not be able to receive the USF if the block is sent towards mobile B and vice versa. Although it would be desirable to transmit data in multiple beams, due to hardware limitations this may not be possible. In addition, EGPRS/GPRS networks transmit using a fixed frame structure. Accordingly, a base station has to transmit the entire block (4 bursts) in the same beam. Hence, mobile A will not be able to detect the USF in the block which also contains data intended for mobile B if the block is transmitted in beam 420. Further, if the data block containing the USF were to be transmitted in beam 410, mobile B would not be able to detect the data in the block.
Accordingly, it would be desirable to obtain the benefits of an adaptive antenna system in a radio network which operates according to EGPRS/GPRS.
These and other problems, drawbacks and limitations of conventional techniques are overcome according to the present invention by a method and apparatus for transmission of data to mobiles which improves the reception of transmitted data in a radio network which uses adaptive antennas.
In accordance with one aspect of the present invention a radio network which employs adaptive antennas for transmitting data blocks determines whether information is to be transmitted to a first mobile. The radio network then determines whether the first mobile and a second mobile are located in a same antenna beam if the information is to be transmitted to the first mobile. If the first and second mobile are located in different beams and the block contains information for both mobiles the data is transmitted toward the first mobile and the data intended for the second mobile is coded so that the decreased antenna gain is eliminated. This is performed on block basis so that if granularity 4 is used the three block only containing data for one mobile will not be received by the other mobile.